Dissolution testing is often performed as part of preparing and evaluating soluble materials such as pharmaceutical dosage forms (e.g., tablets consisting of a therapeutically effective amount of active drug carried by an excipient material). Typically, dosage forms are dropped into test vessels that contain dissolution media of a predetermined volume and chemical composition. For instance, the composition can have a pH factor or acidic concentration suitable for emulating a gastro-intestinal environment. Dissolution testing can be useful, for example, in studying the drug release characteristics of the dosage form or in evaluating the quality control of the process used in forming the dose. In order to ensure validation of the data generated from dissolution-related procedures, dissolution testing is often carried out according to guidelines approved or specified by certain entities such as United States Pharmacopoeia (USP), in which case the testing must be conducted within various parametric ranges. Important parameters include dissolution media temperature, the amount of allowable evaporation-related loss, and the use, position and speed of agitation or dosage-retention devices. Recent developments in robotics and other automating means have been applied to dissolution media preparation and sample analysis technology, and have resulted in improved procedural efficiency and data quality.
As a dosage form is dissolving in the test vessel of a dissolution system, samples of the solution can be taken at predetermined time intervals and transported through a pumping system to the cuvette or sample vial of analytical equipment. The analytical equipment determines drug concentration and other properties. The dissolution profile for the dosage under evaluation—i.e., the percentage of drug dissolved in the test media at a certain point in time or over a certain period of time—can be calculated from the measurement of analyte concentration in the sample taken. The types of analytical equipment commonly provided include those adapted for effecting analytical techniques such as high-performance liquid chromatography (HPLC) and spectral analysis. HPLC entails separating the chemical compounds of the sample for discrete analysis by a detection device (which may be a simply designed UV spectrometer). Flow cells can be used in conjunction with HPLC as shown, for example, in U.S. Pat. No. 4,886,356 wherein Z-type flow cells are disclosed. As one example for implementing spectral analysis, a spectrophotometer uses ultraviolet (UV) and/or visible light to scan the sample and calculate light absorbance values. In one specific method involving the UV or UV-vis spectrophotometer, the UV sipper method, the sample is transferred to a flow cell contained within the spectrophotometer, is scanned while residing in the flow cell, and is then returned to the test vessel. The sample return step is advantageous in that it significantly reduces any analytical errors potentially resulting from a volumetric reduction in the solution still being developed in the test vessel. In general, spectrophotometric techniques are considered to be easier to implement than HPLC techniques for many applications.
The concentration of a given analyte in a sample through spectrochemical determination typically involves several steps. These steps can include (1) acquiring an initial sample (e.g., providing a dissolution testing apparatus with a dosage form such as a drug tablet that has been manufactured from a bulk material, or conducting chromatography, dialysis, and so on); (2) performing sample preparation and/or treatment to produce the analytical sample (e.g., dissolving the dosage from in dissolution media, and possibly adding reagents or pH factor-modifying agents, thereby creating a formulation suitable for measurement or detection by certain instruments); (3) using a sample introduction system to present the analytical sample to the sample holding portion of a selected analytical instrument (e.g., transferring the sample to the sample-holding portion of a UV spectrophotometer); (4) measuring an analytical signal (e.g., an optical signal) derived from the analytical sample; (5) establishing a calibration function through the use of standards and calculations; (6) interpreting the analytical signal; and (7) feeding the interpreted signal to a readout and/or recording system.
Conventional equipment employed in carrying out the above processes are generally known in various forms. Measurement of the analytical signal involves employing a suitable spectrochemical encoding system to encode the chemical information associated with the sample, such as concentration, in the form of an optical signal. In spectrochemical systems, the encoding process entails passing a beam of light through the sample under controlled conditions, in which case the desired chemical information is encoded as the magnitude of optical signals at particular wavelengths. Measurement and encoding can occur in sample cells, cuvettes, flow cells, and other sample containers of various designs. Flow cells permit increased sample throughput and facilitate the automation of filling and cleaning procedures. Test media and calibration media can be pumped or otherwise transferred into the flow cell, and the flow stopped for conducting an absorption measurement. After the measurement is taken, the pumping rate can be adjusted, and the liquid flow adjusted or reversed as needed, so as to remove the entire sample from the flow cell. The flow cell and associated liquid conduits can then be rinsed and another sample introduced into the flow cell. Flow cells can also be utilized to take absorption measurements on flowing streams of analyte-containing media, thereby making the measurement or analysis time-dependent. In this latter case, the flow rate and data acquisition time are controlled to ensure that the absorbance value is obtained for the sample at the proper time.
In addition, a suitable optical information selector must be used to sort out or discriminate the desired optical signal from the several potentially interfering signals produced by the encoding process. For instance, a wavelength selector can be used to discriminate on the basis of wavelength, or optical frequency. A radiation transducer or photodetector is then activated to convert the optical signal into a corresponding electrical signal suitable for processing by the electronic circuitry normally integrated into the analytical equipment. A readout device provides human-readable numerical data, the values of which are proportional to the processed electrical signals.
Considering all of the physical events that must occur over the course of sample preparation and analysis, adequate procedures for calibration or standardization of the system are usually required. For example, standards of known concentration can be introduced at one or more points along the liquid flow circuit of the system. Calibration data can thus be generated, stored and used as part of the analyzing process. Modern calibration procedures are often controlled by computer software. Indeed, a computer-controlled system can be provided to interface with many of the various components of the sample preparation and analysis systems. Such programmable systems are useful for monitoring and coordinating the various hardware operations, as well as for processing both the test data and the calibration data.
For spectrophotometers operating according to UV-vis molecular absorption methods, the quantity measured from a sample is the magnitude of the radiant power or flux supplied from a radiation source that is absorbed by the analyte species of the sample. Ideally, a value for the absorbance A can be validly calculated from Beer's law:
  A  =                    -        log            ⁢                          ⁢      T        =                  -        log            ⁢              P                                            P              0                        =            abc                    ,                    where T is the transmittance, P0 is the magnitude of the radiant power incident on the sample, P is the magnitude of the diminished (or attenuated) radiant power transmitted from the sample, a is the absorptivity, b is the pathlength of absorption, and c is the concentration of the absorbing species.
It thus can be seen that under suitable conditions, absorbance is directly proportional to analyte concentration through Beer's law. The concentration of the analyte can be determined from the absorbance value, which in turn is calculated from the ratio of measured radiation transmitted and measured radiation incident. In addition, a true absorbance value can be obtained by measuring a reference or blank sample and taking the ratio of the radiant power transmitted through the analyte sample to that transmitted through the blank sample.
Ordinarily, the sample is transferred to a sample cell that is contained within the analytical instrument (e.g., spectrophotometer) itself. An example of a conventional dissolution testing system is disclosed in U.S. Pat. No. 6,060,024. Samples are taken from test vessels and, using sampling pumps, carried over sampling lines and through sampling filters. The samples are then transported either to a UV analyzer containing six cells, to an HPLC system, or to a fraction collector.
U.S. Pat. No. 6,002,477, commonly assigned to the owner of the present application, discloses a spectrophotometer that contains a sample cell and a reference cell. A pulsed light source such as xenon flash tube emits very short, intense bursts of light that, after possibly being redirected by one or more reflective surfaces, passes through the entrance slit of a monochromator. After encountering one or more other reflective surfaces, gratings, and apertures or slits, the incident light beam is divided by a fixed beam splitter into two beams having a predetermined intensity ratio. One of these beams passes through the reference cell, and the beam transmitted from the reference cell is received by a reference detector. The other beam passes through the sample cell, and the beam transmitted from the sample cell is received by a sample detector. Provision is also made for measuring the dark signal, which is a measurement of the signal when no light from the light source reaches a detector. The sample, reference, and dark measurements are used to accurately calculate the absorbance of the sample. In another embodiment, the pulsing of the light source is synchronized by control means with the rotation of a carousel. The carousel holds several sample cuvettes, such that its rotation brings each cuvette into position at the sample cell in a step-wise manner.
Other examples of UV-vis spectrophotometers are those available from Varian, Inc., Palo Alto, Calif., and designated as the CARY™ Series systems. In particular, the Varian CARY 50™ spectrophotometer includes a sample compartment that contains a sample cell through which a light beam or pulse passes. Several sizes of sample cells are available. In addition, the spectrophotometer can be equipped with a multi-cell holder that accommodates up to eighteen cells. A built-in movement mechanism moves the cells past the light beam.
U.S. Pat. No. 4,279,860 discloses a multiple injector flow-through dissolution cell designed to handle dosages that have very high dissolution rates. A plurality of flow channels can selectively provide different specimens for the dissolution cell. The dissolution cell itself includes a mixing paddle, and thus functions as the test vessel for the dissolution system. A sample from the dissolution cell is sent through an output line to a spectrophotometer for both measurement and analysis. Fiber-optics are not employed at the dissolution cell.
U.S. Pat. No. 4,431,307 discloses a cuvette-set matrix containing an array of cuvettes adapted for use in measurements using light beams. Each cuvette is provided with a bottom optical window. All other portions of each cuvette are impervious to light in order to prevent the radiation directed into a particular cuvette from disturbing measurements taken in adjoining cuvettes. The cuvette-set matrix is adapted to receive a matrix of measurement beams containing a plurality of sources of measurement beams, such that one source of measurement beams is associated with each cuvette. A detector matrix is disposed on the side of the cuvettes opposite to the side at which the matrix of measurement beams is disposed. Thus, for each cuvette, the measurement beam emitted from the source passes through the liquid contained in the cuvette, through the optical window of the cuvette, and into the detector associated with the cuvette.
Ordinarily, the sample is transferred to a sample cell that is contained within the analytical instrument (e.g., spectrophotometer) itself. In other recently developed systems, fiber-optics are being used in conjunction with UV scans to conduct in-situ absorption measurements—that is, measurements taken directly in the sample vessels of either dissolution test equipment or sample analysis equipment. Fiber optic cables consist of, for example, glass fibers coaxially surrounded by protective sheathing or cladding, and are capable of carrying monochromatic light signals. There have been some applications of fiber-optics in the pharmacological testing industry. In particular, some fiber-optic sampling techniques have been employed as part of dissolution testing. In conventional fiber-optic techniques, a fiber-optic probe is placed directly into the dissolution media and hence its method is described as “in-situ”. Unfortunately, particulates in the media tend to interfere with the UV scan and consequently produce inaccurate data. Appropriate software programs can be used to compensate for the inconsistencies caused by the particulates. However, because each drug sample (e.g., tablet) has unique particulate features, every sample being tested requires a separate algorithm for correcting the errors caused by the particulates of the tablet. Moreover, fiber-optic probes induce turbulence in the dissolution media. Current fiber-optic techniques are also disadvantageous in that they require calibration prior to each test run. First, “standard” media must be put into a test tube and placed over the fiber-optic probe. Second, “blank” media” must be put into a test tube and placed over the fiber-optic probe. The test is then initiated and the UV data are acquired.
One recent example of an in-situ fiber-optic method associated with dissolution testing is disclosed in U.S. Pat. No. 6,174,497. This method involves submerging a dip-type fiber-optic UV probe in test media contained in a dissolution vessel, and keeping the probe submerged over the course of the dissolution run. Several probes can be operatively associated with a corresponding number of test vessels, with each probe communicating with its own UV spectrometer. The probe can be disposed within the shaft of an agitation device in order to reduce effects related to flow aberration, since only the mixing shaft/dip probe combination resides in the test vessel. A light beam (UV radiation) provided by a deuterium lamp is directed through fiber-optic cabling to the probe. Within the probe, the light travels through a quartz lens seated directly above a flow cell-type structure, the interior of which is filled with a quantity of the test media. The light passes through the test media in the flow cell, is reflected off a mirror positioned at the terminal end of the probe, passes back through the flow cell and the quartz lens, and travels through a second fiber-optic cable to a spectrometer. Thus, only the light beam, and not the sample, is removed from the test vessel during the procedure.
The probe disclosed in U.S. Pat. No. 6,174,497 is intended to reduce analytical errors and noise sources associated with conventional techniques requiring the removal of media from the test vessel. Such analytical errors can result from operator errors, programming errors, equipment malfunctions, contamination, clogging, media loss, and so on. This arrangement, nevertheless, requires the use of software algorithms to correct for noise-related physical events. Moreover, the fact that the probe is constantly submerged means that hydrodynamic influences can still affect the release rate of the dosage formulation being tested. While the position of the probe within the test vessel could be controlled by a sampling manifold, providers of this particular design recommend that the probe be maintained in at least a partially submerged position to eliminate the occurrence of air bubbles and fouling due to drying. Furthermore, the fact that the probe remains immersed within the contents of the test vessel means that analytical errors can result from the interference of particulates in the media being detected by the probe, as there is no provision for filtering such particulates from the media.
Another recent example of an in-situ fiber-optic method associated with dissolution testing, available from LEAP Technologies, Inc., utilizes a U-shaped dip probe that is inserted into a test vessel. One leg of the U-shaped probe contains a source optical fiber and the other leg contains the return optical fiber. A gap between the ends of the fibers is defined at the base of the U-shape, across which the light beam is transmitted through the media of the test vessel.
U.S. Pat. No. 5,005,005 also discloses a U-shaped optical-based sensor. The sensor is constructed by forming a U-shaped loop section in a single fiber-optic cable, retaining the shape of the loop in a support structure, and removing the sheathing and coating materials of a section of the cable corresponding to the curved section of the U-shape. This removal creates an exposed fiber core section through which light can be transferred. One end of the cable communicates with a light source while the other end communicates with a photodiode. Ice detection, soil moisture detection, underground tank leak detection, and fluid level sensing are disclosed as applications of the sensor.
For the previously described Varian CARY 50™ spectrophotometer, a fiber-optic dip probe coupler is available to enable in-situ sample measurement methods and effectively replace the need for a sipper accessory. This fiber optic coupler can be housed in the spectrophotometer unit in the place of the conventional sample cell. The coupler includes suitable connectors for coupling with the source and return optical fiber lines of a remote fiber-optic dip probe. The light beam from the light source of the spectrophotometer is directed to source line of the dip probe, and the resulting optical signal transmitted back to the spectrophotometer through the return line.
Fiber optics have also been employed in connection with sample-holding cells. For example, U.S. Pat. No. 6,069,694 discloses a flow cell having two fiber-optic cable assemblies that are spaced apart on opposite sides of the flow cell. Each cable assembly terminates at a distal end that requires the use of either a sapphire window or a lens. The path length between the distal ends is adjustable. The liquid to be analyzed flows through the flow cell between the opposing ends of the cable assemblies. The light passing through the flow cell is carried over one of the cable assemblies to an infrared analyzing instrument.
U.S. Pat. No. 5,715,173 discloses an optical system for measuring transmitted light in which both a sample flow cell and a reference flow cell are used. Light supplied from a light source is transmitted through a collimator lens to a pair of condenser lenses. One part of the light travels through an optical fiber to the sample flow cell, while the other part of the light travels through a second optical fiber to the reference flow cell. On the input side of each flow cell, the respective optical fiber terminates at a collimator lens. On the output side of each flow cell, light transmitted through the cell enters an optical fiber through a condenser lens. The path of transmitted light from each flow cell is directed toward an optical detector, and is controlled by an optical path switcher in the form of a light selecting shutter or disk.
Another example of an optical measurement device is disclosed in U.S. Pat. No. 5,077,481, in which the measurement device is inserted into the liquid sample cup of a spectrophotometer. The device is cylindrical and defines an internal cavity accessible by three lateral openings. Send and return optical fibers are situated above the cavity, and a concave reflection device is situated below the cavity. When inserted into the liquid sample cup, liquid contained in the cup is admitted into the cavity via the lateral openings. A light beam from one of the optical fibers passes twice through the liquid residing in the cavity, since it is reflected off the reflection component, and subsequently is transported away from the measurement device through the other optical fiber.
U.S. Pat. No. 5,428,696 discloses a fiber-optic sample analyzing system in which a plurality of cuvettes each have a source optical fiber and a return optical fiber, with the terminal ends of the fibers requiring the use of light-directing lenses. A device is provided for selecting a source fiber to receive passed radiation through a selected sample of one of the cuvettes, and for returning transmitted radiation from the selected cuvette through a selected return fiber to a spectrophotometer.
U.S. Pat. No. 4,528,159 discloses a sample analysis system in which a belt containing a series of disposable reaction cuvettes is driven along a track so as to guide the cuvettes through several analysis stations. A separate photodetector tube is required for each analysis station. Light guides are used to transmit light from a light source, through filter wheels, through the reaction compartments of the cuvettes, and to the photodetectors.
U.S. Pat. No. 5,804,453 discloses a system in which a fiber-optic biosensor probe is inserted into a test tube. The probe receives a light beam from a light source and sends a testing signal to the photodetectors of a spectrometer. Time division multiplexing and demultiplexing are implemented to distribute light to and from several biosensors. Switching among inputs and outputs is controlled by an input control signal provided by an electronic clocked counter.
U.S. Pat. No. 5,580,784 discloses a system in which a plurality of chemical sensors are associated with several sample vials and arranged between a light source and a photodetector. Optical fibers are used to direct radiation into each sensor, as well as to direct emissions out from the sensors. A wavelength-tunable filter is combined with an optical multiplexer to direct radiation serially to each sensor through the fibers.
In view of the current state of the art, it would be advantageous to provide improved flow cells and flow cell structures (or any other similar type of structure adapted for sample measurement) that are designed and arranged in a manner conducive to high-quality dissolution testing, and that cooperate with fiber-optic components. In operation, such flow cells would not be inserted into the vessels in which dissolution is effected. At the same time, however, the flow cells and their associated liquid flow and fiber-optic components would not detrimentally affect data acquisition, measurement and analysis. It would also be advantageous to provide a liquid flow system adapted for use in conjunction with the improved flow cells, and that would enable improved sampling and calibration procedures.